What’s worse for a plane: a drone or a goose? To answer that question, you’ve got to destroy a lot of fake drones and a lot of fake geese. First things first, the facts: anything heavier than 60 grams that hits a plane risks causing damage, but exactly how much depends on how the bird, or drone, is flying. And that’s where it gets complicated.

Compared to birds, drones are a small but growing issue for planes. The International Civil Aviation Organisation (ICAO) reported 65,139 bird strikes between 2011 and 2014. Figures for drones are harder to come by, but in the UK alone there have been 138 reported incidents involving drones and civil aircraft.

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The difference between a bird and a drone? “Nobody tells a bird it can't be in the air,” says Philippa Oldham, head of transport and manufacturing at the Institution of Mechanical Engineers. But we can, and do, control drones.

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Nevertheless, at 6.10pm on Sunday afternoon, the runway at Gatwick Airport was closed after a drone was seen flying nearby. Five flights were redirected to other UK airports, causing disruption and delays. When it comes to drones hitting planes, it’s the fear of the unknown that’s most alarming.

“There's lots of different modelling happening through the CAA, through the military, but the big challenge is the number of different factors that affect what the impact will be,” Oldham says. “That's everything from the size of it, the mass of it, the speed at which it's travelling and the angle at which it impacts. That, at the moment, is the real problem: what will the real damage be?” To work that out, research institutes and aviation authorities around the world have spent years running simulations.

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When it comes to bird strikes, experts are well aware of the threat. On January 15, 2009, US Airways Flight 1549, an Airbus A320-214, struck a flock of Canada geese three minutes after taking off from New York City's LaGuardia Airport. The plane lost all engine power and had to ditch in the Hudson River off Midtown Manhattan. There were few serious injuries, but it was a stark reminder of the danger posed by something over which we have no control.

Using bird strikes as a model for drones is, obviously, flawed. A full-grown male Canada goose weighs 6.5kg; a DJI Phantom 4 weighs 1.3kg. And unlike a bird, a drone has mechanical and electrical parts. Which is why both require detailed and ongoing simulations and modelling. “That's exactly what they're trying to model. As a drone goes through an engine it's broken down and that will impact or damage the engine as it gets rejected out,” says Oldham. The lithium ion batteries used in drones also pose a risk. A bird is unlikely to catch fire, but a drone might. Conversely, drones don't tend to fly in flocks, whereas birds do. That, combined with the added heft of some birds, makes them a far greater danger. Ultimately, it isn't a comparison game. If your plane is hit by a drone, that's bad. If your plane is hit by a goose, that's bad too.

The canopy of an F16 after a bird strike

Air Combat Command

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As with all challenges in aviation design, Oldham suggests that airplane design may eventually have to be altered in response to the growing, albeit small, threat of drone collisions. “The design of a jet engine is very complicated, but we're always looking at how we can improve its efficiency, reduce its emissions,” she explains. “There are always design improvements going on and the safety and risk that comes out of impacts with drones will feed into that design process.”

While the Civil Aviation Authority’s Drone Code states people must not fly drones near airports, airfields or aircraft, some still take the risk. Five of the close encounters with drones reported so far this year have been placed in the highest risk category, meaning a “serious risk of collision” existed.

Tackling the drone issue through legislation or changes to drone and airplane design might be the best solutions. Despite the growing number of drone incidents, prosecutions remain elusive. In September 2015 a man was fined £1,800 and barred from flying or owning a drone for the next two years after posting videos on YouTube of him flying drones over Premier League stadiums. People caught using drones to smuggle drugs and over contraband into prisons have also been prosecuted. “It's obviously a massive challenge that we've got with drones,” says Oldham.